The invention disclosed relates in general to switching devices and in particular to circuits for suppressing arcs which may occur upon switch operation.
There is a significant need for controlling high voltage direct or alternating currents with a physically small switching device, such as a relay. The problem involved in satisfying this need, however, is that as the contacts of a relay are opened, an electrical discharge occurring where current flow is interrupted causes heating and burning of the electrodes, leading to welding and destruction thereof. One attempt to solve this problem is disclosed in U.S. Pat. No. 4,438,472 to Woodworth, as depicted in FIG. 1 therein, wherein the contacts of a switch S1 are shunted by a bipolar transistor Q1 for diverting a load current around the mechanical switch when the contacts are opened. Such current is diverted long enough to enable the contacts to be separated by a distance sufficient to prevent arcing.
The transistor in Woodworth is turned on as the switch opens as a result of current applied to the base of the transistor through a biasing capacitor C1, and is turned off after the contacts become widely separated when the biasing capacitor has charged. Arcing is avoided when contact bounce occurs upon closure of the contacts by providing a diode D1 connected in parallel with the base-emitter portion of the circuit. Diode D1 discharges the biasing capacitor upon the first closure of the contacts, permitting flow of current to the transistor base as the contacts thereafter bounce apart for again turning on the transistor and shunting the arcing current around the contacts.
While the arc suppressing circuit disclosed by Woodworth functions adequately in some applications, it suffers from drawbacks making it impractical in others. First, since the base input impedance of a bipolar transistor is relatively low, the size of the biasing capacitor must be fairly large in order for the transistor to stay on long enough to provide arc suppression for an adequate contact separation time. For example, Woodworth discloses a typical application wherein a one microfarad biasing capacitor is necessary to permit the transistor to remain on for a period of one millisecond. The large capacitor adds expense to the device and increases the size of the packaging required, making the Woodworth arc suppression circuit less suitable for use in conjunction with micro-relays where small packaging is necessary.
Another drawback associated with the Woodworth suppression circuit relates to the relatively high active state collector-to-emitter impedance of the typical bipolar transistor. During arc suppression, the current shunted through this high impedance path generates waste heat which can cause equipment failure, particularly when the contacts are opened and closed frequently. Additional heat sinking provisions, necessary to permit high duty cycle operation of the transistor, can cause further increases in packaging size and expense.
Finally, the Woodworth suppression circuit is only suitable for use in conjunction with direct current switching operations.
What is needed, and would be useful, is a contact arc suppression circuit which could be implemented in a small package, which would generate little heat during arc suppression and which could be used in conjunction with either AC or DC current switcing applications.